Kart's CG and what it does

24 Oct

Chanced upon this article.
Informative, and relatively easy to understand.
Now, I hope I can fully comprehend and applied the good
knowledge.
----

You hear a lot about center of gravity (CG), when dealing with kart setup.
But what does it mean and what is really going on?

Well hold on, because I am about to tell you.

First off, what exactly is a CG.

Simply put the CG is the point by which all the forces on the kart act through.
It’s an imaginary point where if you attached a string to it the kart would be perfectly balanced and you could rotate it in all directions. In general, this point is somewhere right in front of your stomach or chest. If you flop around in your kart a lot as you drive the point can move which will alter your handling. Some karters will actually move around on purpose to give them an advantage. For instance, you can lean back as you brake to put more weight on the rear tires so you can brake harder.

So what does it mean that all the forces act through this point? Ok, let’s say your rear weight bias is %60. That means that the CG is at %60 of the distance from the front to the rear tires. Now when you go through a turn the lateral forces act through this point so 60 percent of the force goes to the rear tires and 40 percent goes to the front.

This explains the front to rear location of the CG, but there is also a vertical component – how high above the ground the CG is located. There are ways to calculate this based on tilting the kart, but it’s kinda complicated and in my opinion, it’s not necessary to know exactly anyway. What you do need to know about the vertical component of the CG is that it has a profound effect on the grip of the kart. Here is where one of the motorsport oddities unique to karting shows its head. The front of the kart acts as a car, but the rear acts as a motorcycle? Say what? Well it is easier to explain by showing you.


If you remember that the rear of a kart corners on only the outside wheel, you might be able to figure out what I am talking about real quick. The pictures depict the CG acting through the tire. Motorcyclers have a pretty good advantage in this sense in that they can lean to the perfect angle necessary and their tires are round. A kart however has a flat tire and cannot lean. Why does this matter? Well what happens when a motorcycle corners faster. It must lean more. Why is that? Because as the lateral force increases the center of gravity must act at a smaller and smaller angle to the contact patch otherwise the rider will tip over. If he leans too much he will slide the rear tire out. A good rider can naturally feel this balance.

On a kart however, in order to get this maximum amount of grip you must set the kart up properly beforehand to get this ideal angle. So looking at the picture it is pretty obvious you can either move the center of gravity (you) or the tire. Just like the bike, there is a perfect balance point that will give you the maximum grip. If you 'move the tire out' or the CG down you will lose grip and start sliding more. If you move the CG up or 'the tire in' the kart will want to lift up and flip over. This rarely happens for reasons I will explain in a second. The importance of all this is that changing grip levels based on track, tires, and many other things require that you either move the CG or rear tire to maximize grip, which is one of the reasons setting up karts is so specific to drivers and tracks

I also mentioned the rounded profile of the motorcyle tire. Karts have a flat profile, so what happens is that if you move the CG too high or the tire in too much the kart will want to tip over, but as the kart starts to lift, the outside tire is no longer flat and you lose grip really fast and the inside tire comes crashing back down. It then regains grip and the kart starts to lift again. The repeating cycle of this is one of the main causes of hopping karters experience. So my recommendation is to find the balance point where your kart is just shy of wanting to hop. In general This is where you will have maximum grip at the rear.

Ok, so now we understand how the rear of a kart is like a motorcycle, how is the front like a car? Well this is the easy part of kart setup. In order to maximize front grip you want to get the CG as low as possible. Why is that? Well it is all because of load sensitivity, which I explain in my tire article. Basically the lower the CG the less weight transfers the outside tire during cornering and the more evenly loaded the front two tires are. The more evenly loaded the two tires are the more grip they will have.

The take away message is to get your CG as low as possible. This gives maximum grip at the front and at the rear you can just move the tires in until you find your perfect balance point. A side advantage of this is that your rear track will be narrower and you will be able to use more of the track to improve your cornering speed. There are other effects of lowering the CG other than the obvious butt scraping that could occur, that we will address shortly, but these can be tuned around using other methods.

So now we now what moving the CG up and down does, but what about moving it forward and backward. Karters often think there is some magic number for the front to rear percentage that is ideal. Well just like most everything else there is always a compromise. Basically you are compromising between your pure lateral grip (cornering) and longitudinal grip (acceleration and braking). First let’s discuss lateral grip. If you were doing a skidpad competition where you just drive in a circle the ideal front to rear weight percentage would about equal the relative tire contact patch size. Here is where tire load sensitivity comes in again.

Moving weight back creates more relative grip in the front and moving weight forward gives more relative grip in the rear. This is backwards to the way most people think, but nonetheless it is absolutely true. You could optimize your balance by driving in a circle around a 100 foot skidpad and move weight forward or back until you maximized your speed. In fact in the lower powered classes this might be a good strategy because the karts do not have much acceleration potential anyway. In a kart with more power however you could benefit from running a more rearward weight bias. Think of a dragster where all the weight is over the rear tires. This is not just related to the relative power of the kart either, it is also related to the track you are running. Even a very low powered kart can generate some wheelspin coming out of a super tight hairpin and likewise a high powered shifter is not going to be doing much acceleration during a 100 mph corner on a road course. Braking works in a similar manner. On a kart with rear only brakes you want as much weight as possible on the rear so you can brake harder. On a kart with brakes all around you also want a rearward weight bias because during braking weight shifts forward and a rearward weight bias allows the tires to work more evenly and thus create more overall grip. Hopefully you can see the compromises involved in moving weight forward and backward. Go more forward if you are looking for ultimate cornering ability, and go more backwards if you are generating excessive wheelspin on corner exit. If you want to really put your knowledge to use, try leaning forward and backwards at the right time to maximize your grip.

I focus mostly on sprint karting, but for those curious using a leftward bias for circle track racing will generate more grip at front and will allow you to run your rear track narrower. There actually is a point where you can run too much left bias, but in practical use you will probably never reach it.

There is another affect of moving the CG I want to go over and it is related to kart jacking (unloading the inside rear tire). If you look at the photo to the right you will see that I have drawn a line from the left front tire to the right rear. This is depicting a kart in a left hand turn and the way the CG acts on a pivot point created by these two tires. Basically the further to the CG is moved forward the stronger the jacking forces are on the kart and the more the inside rear will unload. This is why a lot of people suggest moving weight forward if you are understeering, you may have not been unloading the inside rear enough and this accomplishes that, but this is not the best way to tune a kart. Move your CG to maximize traction and then alter your jacking through other methods. Now that you understand that I am going to confuse you some more. The higher the CG is, the greater this jacking force as well. If you can picture in three dimensions how the CG is a lever arm attached to this virtual pivot arm you can visualize how this works. You may also notice that moving the tires in and out will change where this virtual pivot arm is.

Lots of things have multiple affects when they are changed, but you can often separate them into overall grip and jacking effects. I always say maximize your grip first because you can almost always tune the proper amount of jacking through other means.

from http://howkartswork.com/?p=299